In a process such as Ugine-Sejournet extrusion process or Mannesmann tube making process via a rolling or forging process, for manufacturing a seamless steel pipe from a billet, produced by continuous casting as a raw material, for instance, the inner part of the billet of use constitutes the inner surface of the pipe. Therefore, the billet for manufacture of a seamless pipe is imperatively required to be homogeneous in quality not only on the outer surface but also on the inside and, therefore, the quality control of the inner part of the billet is important. If center porosity occurs in a billet obtained by continuous casting and the extent thereof is above a tolerance limit, the seamless steel pipe produced from the billet often have inner surface defects, which are likely to be rejected from the quality viewpoint.
Therefore, a secondary cooling method utilizing thermal shrinkage during billet cooling has been proposed for the purpose of reducing the possibility of center porosity occurring in the billet in a continuous billet casting process.
For example, in Japanese Patent Application Publication S62-61764, there is disclosed a method which comprises subjecting the billet surface to forced water cooling, following the direction of casting, in a region ranging from the site at 2-15 m in front of a liquid core crater end inside the billet in the casting direction to the liquid core crater end to an extent that the shrinkage thereof during solidification at least comes to the amount of shrinkage in volume to cause shrinkage of the billet's solidified shell and thus reduce the billet cross section, thereby reducing the extent of center segregation.
Further, in Japanese Patent Application Publication S62-263855, there is disclosed a method which comprises successively lowering the billet surface temperature, following the direction of casting, in an area spanning from the site at 2-15 m in front of a liquid core crater end inside the billet in the casting direction to the liquid core crater end, to a temperature not less than the A3 transformation temperature of the steel or the starting temperature TA of Acm transformation and not more than the effective billet surface temperature Tv given by Ta+(TN−Ta)×0.3=Tv in response to the progress of solidification of the billet liquid core to cause the billet solidified shell to shrink and thus reduce the billet cross section and thereby reduce the possibility of center porosity formation. In the above equation, TN is the billet surface temperature resulting from open air cooling after leaving the pinch roll unit and Ta is the billet surface temperature at which such average cooling of the solidified shell that is necessary for compensating the amount of shrinkage during solidification is attained.
Further, Japanese Patent Application Publication H02-15856 discloses a method which comprises subjecting the billet to forced cooling, while the core of the billet during continuous casting is in a soft solidified phase condition, so that an effect such that the soft core is always compressed by the already completely solidified shell around the core owing to the difference in thermal shrinkage between the core and the shell, to thereby reduce the possibility of center porosity formation.
However, the methods disclosed in Japanese Patent Application Publication S62-61764, Japanese Patent Application Publication S62-263855 and Japanese Patent Application Publication H02-15856, among others, have the following problems. For example, (1) when forced cooling is carried out on the side excessively upstream relative to the point of final solidification, no more temperature allowance for cooling remains at a time when the possibility of center porosity formation becomes really high and the cooling effect decreases; (2) if cooling is stopped when the core of the billet is not yet in a solidified state, return of heat causes increased center porosity or internal cracking; (3) the ranges of proper conditions for obtaining the effects of reducing center porosity and center segregation are very narrow, so that extraneous disturbances, for instance, readily cause the actual production conditions to deviate from the proper ranges.
Previously, the present inventors proposed the methods disclosed in Japanese Patents No. 2,856,068, No. 3,405,490 and No. 3,401,785 and summarized below as technologies of improving the methods disclosed in the above-cited Japanese Patent Application Publications S62-61764, S62-263855 and H02-15856.
The method proposed in Japanese Patent No. 2,856,068 is a method of cooling which comprises starting billet surface cooling at a specified density of cooling water at the time of arrival of the solid phase ratio in the central portion of the billet at 0.1-0.3 and continuing water cooling at that density of cooling water until arrival of the solid phase ratio in the central portion of the billet at a level not less than 0.8. The method proposed in Japanese Patent No. 3,405,490 is a method for improving the inner quality which comprises starting surface cooling of a billet having a diameter or thickness not exceeding a specified value with water in a specific amount within a specified range at the time of arrival of the solid phase ratio in the central portion of the billet at 0.2-0.8 and continuing water cooling with the above specific amount of water until complete solidification. The method proposed in Japanese Patent No. 3,401,785 is a method of cooling which comprises adjusting the density of billet surface cooling water to a value within a specified range from a site 0.1-2.0 m in from of the liquid core crater end in the casting direction until arrival of the solid phase ratio in the central portion of the billet at a level not less than 0.99, while increasing the density of cooling water toward the downstream side.
The present inventors have thus brought about marked improvements with respect to the problems (1)-(3) mentioned above by putting the technologies disclosed in the above-mentioned Japanese Patents No. 2,856,068, No. 3,405,490 and No. 3,401,785 to practical use. For obtaining the inner quality improving effects more stably and more reliably, however, there is still room for improvement from the technological viewpoint.